The present invention relates to a hydraulic control system for a hydrokinetic torque converter including a lockup clutch.
As is well known in the art, in a torque converter there is a slippage between the pump impeller and the turbine runner during operations of the torque converter. For this reason, although it is easy to operate a vehicle which is provided with an automatic transmission including the torque converter as a part of a power transmitting train, the fuel consumption of the vehicle is increased. As a solution to this problem, various hydrokinetic torque converters have been proposed in which the pump impeller and the turbine runner are mechanically coupled together to prevent the slippage therebetween in a forward top speed gear range in which the torque multiplying effect of the torque converter is unnecessary and/or at a vehicle speed above a predetermined value at which speed the alteration of the engine torque has no influence on the riding quality of the vehicle.
In a hydraulic control system for a hydraulic torque converter of this kind, a pressure retaining or maintaining valve controls the flow of the hydraulic working fluid delivered from the torque converter to maintain the hydraulic pressure in the interior of the torque converter at a predetermined value, and a lock-up control chamber, a clutch chamber, communicates with a source of pressurized working fluid which supplies the hydraulic working fluid to the torque converter. Supply of the hydraulic working fluid into the lock-up control chamber releases a lock-up condition of the torque converter while exhaust of the hydraulic working fluid from the lock-up control chamber effects the lock-up condition of the torque converter to lock the pump impeller and the turbine runner together.
In this instance, the lock-up condition is attained by a pressure differential between the lock-up control chamber and a torque converter chamber communicating with the interior of the torque converter and is released by eliminating the pressure differential. The value of the hydraulic pressure in each of the torque converter chamber and the lock-up control chamber depends mainly on the length and size of each of the hydraulic working fluid supply passages for the torque converter and the lock-up control chamber. Thus, in some conventional hydraulic control systems for the torque converters, the flow resistance of a hydraulic working fluid supply passage for the lock-up control chamber has been larger than the flow resistance of a hydraulic working fluid supply passage for the torque converter so that the hydraulic pressure in the lock-up control chamber has been lower than the hydraulic pressure in the torque converter chamber when the hydraulic working fluid is fed into the lock-up control chamber. As a result, it has been impossible to surely release the lock-up condition of the torque converter. Accordingly, at this time there has been a risk that the turbine runner is directly rotated by the drive shaft for the pump impeller.